Field of the Invention
The invention relates in general to heaters and, in particular, to a new and useful heater for motor vehicles which can be operated on several power settings.
Such heaters are used as auxiliary heaters or heaters which are in action when the car is not running. Such heaters include e.g. separate heating or add-on heaters in buses, trucks, soil moving vehicles and similar vehicles, and also in automobiles. There are various types of such heaters including heaters using either water or air as the heating medium; other criteria are type and setting of the heater, there are e.g. vaporizing burners, pressure pulverizers and rotary atomizing burners. Another distinguishing factor is the kind of ignition, e.g. by means of a spark plug or a glow plug.
There are certain rules and regulations for the operation of such a heater. One of these regulations is that once the burner has started, and, in particular, after the beginning of the fuel transport, the flame has to burn within 180 seconds. Another regulation requires that if the flame dies during stationary operation, the absence of the flame has to be recognized within 240 seconds so that the fuel supply can be stopped in time to avoid the flow of a large amount of fuel into the burner chamber which represents a potential source of danger.
The above regulations for the operation of the burner necessitate a direct or an indirect flame control. A direct flame control is e.g. achieved by means of an optical sensor, e.g. a photo-transistor delivering an output signal as long as it is illuminated by the bright flame. In some cases, however, it is not possible to install an optical sensor inside the burner chamber. In this case a thermo-element which is not arranged inside the burner chamber, but on the outside of the burner chamber wall or on the heat exchanger could be used. A prerequisite for the use of such a thermoelement is that it will react relatively fast, and therefore, that the burner chamber wall or the heat exchanger wall has good conductive properties, i.e. it is not thermally inert.
The use of thermo-elements on the outside of the burner chamber wall or the heat exchanger wall is problematic in cast metal heat exchangers, which are usually made of aluminum. In these heat exchangers the temperature on the outside of the burner chamber wall or the heat exchanger wall changes only relatively slowly. Up until now a bi-metal switch has been used which opens (or closes) upon starting of the heater once a certain temperature is exceeded and thus indicates the lighting of a flame. When the temperature falls below a certain second point, the bi-metal switch closes (opens) to indicate the absence of the flame. Such bi-metal switches are relatively safe. They are adjusted during the production of the heaters for the respective heater types in dependence on the speed with which temperature changes occur on the outside wall of the burner chamber or the heat exchanger.
The use of these two-position switches, which react relatively slowly, is problematic in heaters which can work on several, e.g. on two power settings. For, if the heater is switched from a relatively high setting to a lower setting, this is followed by a relatively large drop in temperature upon which the temperature is maintained at to a relatively low level. The relatively low temperature level at the "LOW" setting is so low that upon the flame blow-off from the "HIGH" setting the temperature is not reached within the required four minutes. Possibly the low temperature level is even below the switch-temperature for flame recognition.
German patent No. 35 17 953 A1 describes a heater with a control device for the heat setting. The temperature gradient between two temperature thresholds for the heating of the heat exchange medium is determined for control of the heater. The arrangement detects how long it takes the heat exchange medium to be heated up between a first and a second temperature threshold. This period of heating up is compared to a specific set value and, depending on the result of the comparison various ranges of control are passed through. This kind of control is not suitable for a flame control which is capable of registering that the ignition has failed and which can also register a flame blow-off during normal operation.